Not so fast, Dorothy! It looks as if Earth may not be so special after all. Second in a two-part series.

Seth Shostak has long been waiting for that celestial phone call. He's the senior astronomer at Mountain View's SETI Institute, which stands for the Search for Extraterrestrial Intelligence. Since 1960, SETI has been using giant antennae to sweep the sky for transmissions from other worlds.

We Earth folks transmit all the time, albeit accidentally, sending radio, television, and military radar signals out into space. Other intelligent beings might not be able to interpret any of it, but to them our broadcasts should seem distinct from anything in nature  they have deliberate patterns, and show up only at certain spots on the frequency dial. If alien transmissions are out there, Shostak says, we can find them the same way.

After all, the fact that our civilization is only recently technological doesn't mean everybody else's has to be. "We've had radio for a hundred years, but the galaxy is three times as old as the Earth," Shostak says. "There are going to be plenty of planets out there that are several times as old as Earth, so there could be societies out there that in principle could be billions of years ahead of us. For them to build a transmitter and ping our planet might be just a high-school science fair project."

So far, that alien science project hasn't won any prizes. But Shostak points out that SETI has always had to borrow time on others' equipment, so the search has been painstakingly slow. He expects that to change this fall when its new Allen Telescope Array plugs in the first 42 of 350 planned antennas. These will be devoted entirely to SETI and will collect data round the clock, dramatically speeding up the search. Over the past 47 years, SETI has examined 750 star systems. In the next two dozen years, Shostak expects to examine one million.

In a way, searching for proof of intelligent beings in space is a bit like searching for Earthlike planets: It's betting on the odds that in a universe this vast, anything that happened once could happen again.

There is, in fact, a famous math formula that attempts to describe the likelihood of detectable intelligent life existing in the Milky Way. To use the Drake Equation, you multiply a long list of factors, including the number of suitable stars in the galaxy, the presumed number of habitable planets, the fraction of them on which life theoretically forms, and the chances that that life developed communications equipment.

But few of the variables in the Drake Equation have fixed values, so everyone who uses it comes up with a different estimate of how crowded our galactic neighborhood is. Its inventor, radio astronomer and SETI founder Frank Drake, estimated that ten thousand other intelligent, communicating species exist in the Milky Way. Astronomer and pop-culture icon Carl Sagan thought it was more like a million. Geoff Marcy thinks it's just one, and it's us.

Forget all of the other uncertainties that stand in the way of producing life, he says  not just having the right star and the right planet and the right chemicals, and not just making sure that fledgling life survives ice ages and meteor impacts and all the other cataclysms that could befall a young planet. Even if you manage to produce an intelligent society, he asks, how long does it last? Nobody knows, but Marcy suspects the window might be very short. "When a species becomes technological, as we have within a few hundreds of years, they develop weapons, they develop toxins, they develop ways to ruin their environment, they tinker and they make a mistake," he says. "How long do you throw the dice?"

In the end, he thinks, each galaxy may be capable of producing multiple intelligent species that will never coexist; they'll be like a chain of lights on a Christmas tree, each one winking on as another winks off. Even if there's another intelligent species in the galaxy next door, Marcy says, we'll never hear from them because they're too far away.

Shostak thinks this is overly pessimistic, although he certainly agrees there may be a synchronicity problem, in which not all intelligent species are at the broadcasting stages of their existence at the same time. "Earth has had life for somewhere between 3.5 and 4 billion years, and how much of that time did it have life that could build a radio transmitter? The last couple decades," he says. "So you can be sure that if there's lots of life out there that most of it is not building the kind of technology we could find."

But all of these doubts, Shostak points out, can be settled by finding just one positive signal. "For five hundred years, astronomers have been telling us we're not that special," he says. "We used to think the Earth was the center of the universe. That's pretty special. Well, it turns out it's not."

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